Summary of Work: There was a good correlation between immunohistochemistry and direct sequencing for the P53 mutation in DMH induced malignant fibrous histiocytomas (MFH). All of MFH had a characteristic P53 G-A transition, consistent with 06-methylguanine mispairing with thymine; the most common DNA lesion caused by alkylating agents. DMH-induced uterine MFH with P53 mutations also had a higher proliferative rate than other DMH induced sarcomas. Uterine sarcomas were further evaluated for tumor biology markers such as estrogen receptors ad desmin. Our data illustrated that DMH-induced uterine sarcomas are not of smooth muscle origin, and specifically that MFH have unique P53 GC-AT transitions which most likely contribute a proliferative growth advantage to this tumor type. The manuscript for these studies is in preparation.